The present invention relates generally to generation of an LO signal for down conversion of a received RF signal and, more particularly, relates to a method and apparatus for doubling and quadrupling the frequency of an LO signal with superior spur suppression.
In many applications, generation of a local oscillator (xe2x80x9cLOxe2x80x9d) signal for down conversion of a received RF signal is required. Examples include digital broadcast system (DBS) tuners or receivers, cable modem tuners, and a variety of devices operating within wireless communication systems such as a cellular telephones, cordless telephones, wireless handsets, mobile stations and pagers. In these and other applications, the received RF signal usually takes the form of a carrier signal at a carrier frequency that is modulated by an information signal at a baseband frequency. As the baseband frequency is typically much lower than the carrier frequency, the receiver must translate or down convert the carrier frequency of the RF signal down to the baseband frequency in order to extract the baseband information from the signal.
In some systems, the frequency of the RF signal is first converted to an intermediate frequency (IF) and the IF is then converted to the baseband frequency. In other systems, which are referred to as direct translation or direct conversion systems, the RF frequency is converted directly to the baseband frequency. Direct conversion systems are advantageous in that the IF stage is eliminated, thereby reducing the cost and size of communication devices incorporating the systems.
A mixer usually carries out frequency translation or down conversion of RF signals. The mixer combines the RF signal with an LO signal to produce an output signal at the IF or baseband frequency. Often, the local oscillator takes the form of a voltage-controlled oscillator (VCO). In order to convert a receive signal frequency (xcfx89RF) to a desired intermediate frequency (xcfx89IF), for instance, the receive signal is mixed with a sinusoidal LO signal of the form A0cosxcfx890t, where xcfx890=xcfx89RFxe2x88x92xcfx89IF. In a direct translation system the intermediate frequency is equal to zero (xcfx89IF=0), so xcfx890=xcfx89RF.
Direct translation receiver architectures are problematic in that there are likely to be interactions and interference between the RF signal and the VCO signal. Since, for direct conversion, the LO frequency must be equal to the RF signal frequency (i.e., xcfx890=xcfx89LO=xcfx89RF), the receiver may receive its own LO signal (self-interference). Leakage of the VCO signal into the RF signal, or vice-versa, can create spurious signal information and can cause DC offsets in the down-converted signal large enough to saturate subsequent components and prevent amplification of the baseband signal. Conversely, leakage of the RF signal into the VCO can cause frequency pulling and lead to an increase in phase noise, resulting in overall system desensitization.
For these reasons, direct conversion receivers require a significant amount of isolation or shielding between the RF signal and VCO. Alternatively, the VCO output frequency can be kept outside the RF signal bandwidth and passed through a frequency multiplier to generate an LO signal at the RF frequency. In one conventional architecture, for example, the VCO outputs a signal at half of the receive frequency and is then passed through a frequency doubler to generate an LO signal at the RF frequency.
This approach can still be problematic. In some systems such as wideband RF systems, a VCO signal at half the RF frequency may still be within the RF frequency range. The RF frequency band in a digital broadcast system (DBS), for example, extends from 950 MHz to 2150 MHz. Halving the corresponding VCO frequency yields a VCO frequency band extending from 475 MHz to 1075 MHz, resulting in an overlap in the frequency band extending from 950 MHz to 1075 MHz. To down convert a receive signal at 2150 MHz, for example, the VCO frequency would be tuned to 1075 MHz and then doubled to 2150 MHz by a frequency multiplier. Prior to doubling, however, the VCO frequency at 1075 MHz may leak into the down conversion mixer and cause down conversion of signals at 1075 MHz, thereby creating a false baseband signal on top of the properly down converted 2150 MHz RF signal. Moreover, VCO spurs (i.e., harmonics) created by the frequency multiplication may fall within the receive band and may be down converted to create spurious baseband signals.
The subject invention is directed toward an apparatus and method for generating an LO signal that isolates the VCO and RF signals and generates low spurious output. An LO generation circuit is provided that includes a broadband frequency doubler that receives a pair of differential input signals with a given frequency and phase relationship and creates a pair of differential output signals. The frequency of the output signals is twice the frequency of the input signals, but the phase relationship of the input signals is maintained. In addition to doubling the frequency and maintaining the phase relationship of the input signals, spurious noise signals and RF-VCO leakage are suppressed. In some applications, the input signal frequency may need to be quadrupled while still maintaining the same phase relationship of the signals. In this case, a second doubler is used in series with the first doubler. This configuration quadruples the frequency of the original signal and, since the doublers do not alter the phase relationship of the signals, the phase relationship of the original signals is maintained at the output of the second doubler.
In one embodiment of the invention, an LO signal generation circuit for generating an LO signal to down convert an RF signal within an RF bandwidth is provided. The circuit includes a VCO that generates a signal having a frequency within a bandwidth that is a subset of the RF bandwidth. A first phase shifter is coupled to the VCO and converts the VCO signal into a quadrature VCO signal having an in-phase component I and a phase-shifted component Q. A first frequency doubler is coupled to the first phase shifter and doubles the frequency of the quadrature VCO signal while maintaining the phase relationship between the quadrature components. A second frequency doubler is coupled to the first frequency doubler and redoubles the frequency of the signal output by the first frequency doubler to output a signal having a quadrupled VCO frequency. A switch selects one of the outputs from the first frequency doubler or second frequency doubler to serve as the LO signal.
In another embodiment of the invention, an inventive frequency doubler is provided. The frequency doubler receives a differential signal having an in-phase component I and a quadrature component Q and outputs a signal having a frequency that is double the frequency of the input signal and a phase balance that is the same as the phase balance of the input signal.
The frequency doubler comprises four mixer cores. A first mixer core receives the I component of the input signal on two input ports and mixes the I components to generate an I product, and a second mixer core receives the Q component of the input signal on two input ports and mixes the Q components to generate a Q product. Means are provided for subtracting the Q product from the I product to generate the in-phase component of the output signal. A third mixer core receives the I component of the input signal on one input port and the Q component on another input port and mixes the I and Q components to generate an IQ product, and a fourth mixer core receives the I component of the input signal on one input port and the Q component on another input port and mixes the I and Q components to generate another IQ product. Means are provided for adding the two IQ products to generate the quadrature component of the output signal.
The present invention also provides a method for down converting an RF signal. The method comprises the following steps:
dividing the potential RF bandwidth into upper and lower portions;
receiving an RF signal;
if the RF signal is in the lower portion of the bandwidth, generating a VCO signal having a frequency that is one-half the frequency of the RF signal, passing the VCO signal through a phase shifter to yield an in-phase and a quadrature signal, and generating an LO signal by doubling the frequency of the VCO signal while maintaining the phase relationship between the in-phase and quadrature input signals;
if the RF signal is in the upper portion of the bandwidth, generating a VCO signal having a frequency that is one-fourth the frequency of the RF signal, passing the VCO signal through a phase shifter to yield an in-phase and a quadrature signal, doubling the frequency of the VCO signal while maintaining the phase relationship between the in-phase and quadrature input signals, and redoubling the frequency of the VCO signal to generate the LO signal; and
down converting the RF signal by mixing the RF and LO signals.
A method for combining the frequencies of an in-phase signal I and a quadrature signal Q while maintaining the phase balance between the signals is also provided. The method comprises the following steps:
providing an in-phase signal I having a first frequency;
providing a phase-shifted signal Q having a second frequency;
squaring the I signal and the Q signal to generate an I product and a Q product;
multiplying the I and Q signals to generate first and second IQ products;
subtracting the Q product from the I product to generate an in-phase signal Ixe2x80x2 having a frequency equal to the sum of the first and second frequencies; and
adding the first and second IQ products to generate a quadrature signal Qxe2x80x2 having a frequency equal to the sum of the first and second frequencies, wherein the phase relationship between the in-phase output signal Ixe2x80x2 and the quadrature output signal Qxe2x80x2 is the same as the phase relationship between the in-phase input signal I and the quadrature input signal Q.
These and other aspects of the present invention will become apparent in the following description, drawings and claims.